I have the CUSL2-C motherboard from ASUS. It supports up to 512mb total, and DOES support 256mb per slot. It does NOT support ECC. My problem is, each stick is registering as only 128mb. I can not figure this out. I don't think the ECC would cause any problems because if it did, I probably wouldnt be able to start my machine at all. Any ideas? I'm all out.

I was using an ASUS CUSL2-C-BP with 2x 256MB Crucial 2-2-2 DIMM's without a single problem, however mine were the non-ECC versions.
Have you tried posting in the forum's over at www.asusboards.com ?
Really nice bunch of people and as the name suggests, it's a board dedicated to ASUS motherboards - plenty of knowledgeable people over there.

Note: I'm not saying people here at ntcompatible aren't knowledgeable, but in case nobody here has any ideas!

A : The Intel 815E chipset on CUSL2 series cannot support registered SDRAM memory. You will only get half of the amount displayed or no display if you install registered memory. It also cannot support ECC memory. But, you still can use ECC memory on 815E chipset without ECC function.